The growing use of plastic materials to replace conventional wood and metal components in a variety of semi-structural and structural applications poses difficult bonding challenges for adhesives. Primers and surface treatments are often used to provide adequate adhesion, but increase the cost and complexity of the bonding process. It has recently been discovered that adhesives based on organoborane initiators can improve the adhesion to plastics of free-radically cured adhesive compositions. (M. F. Sonnenschein et al., Macromolecules, 2004, 37, 7974-7978).
Due to the pyrophoric nature of organoboranes, they must be complexed to avoid oxidative decomposition in air. Typically, organoboranes are complexed with amines. A number of two part systems have been described in the literature where the complexed organoborane is present in one part and a decomplexing agent is present in the second part. However, these systems still suffer from poor shelf stability or excessively fast reaction. One method to enhance shelf stability is to use a vast excess of amine in the part containing the organoborane to drive the equilibrium toward the complexed material. Nitrogen/boron ratios above 4 and as high as 34 have been proposed. (Webb et al. U.S. Pat. No. 6,740,716 B2). Such high ratios improve stability, but may still require exotic stabilizers such as hydroxyl amines or nitrile oxides to avoid premature free radical polymerization if a free radically polymerizable component is present in the part containing the organoborane. (Jialanella et al. US 2005/0004332 A1, and Proceedings of the 30th Annual Meeting of the Adhesion Society, 2007, p. 147). In addition, when isocyanate is used as the decomplexing agent, its rapid reaction with the amine, particularly at high levels of amine, can cause the adhesive to get very hot and the reaction to proceed too fast. Excessively fast reactions do not provide sufficient time for the adhesive to form a strong bond with the substrate. (Pocius, U.S. Pat. No. 6,093,778 (col. 13, line 52) and Webb et al., U.S. Pat. No. 7,235,617 B2 (col. 16, lines 4-47)). To overcome this problem, high levels, ˜40%, of high heat capacity fillers have been employed. (Webb et al. U.S. Pat. No. 7,235,617). However, use of high levels of filler can lead to separation during storage and create difficulties in dispensing the adhesive.
Therefore the need exists for a two part adhesive system with good stability, high adhesion to untreated plastic substrates, and good processing characteristics in conventional equipment. The invention fulfills this need.